[keyboard applet] Disable by default on X11

If the applet isn't ready to be a replacement on X11 disable by default. This means the KCM settings still work as before.

For wayland we've disabled the relevant SNI in kwin so need something in this release.

This detects if we're on wayland and automatically enable then. This gives us the best of both worlds for Plasma 5.21.

Alternate to the other two patches: !297 (closed) !312 (merged).

Requires corresponding obvious kwin patch.